1. Field of the Invention
This invention relates generally to a vertical deflecting circuit for a television receiver, and is particularly useful for a vertical deflecting circuit for a television receiver which is used to project an image produced on its cathode ray tube onto a screen through an optical system such as a lens.
2. Description of the Prior Art
In the prior art there is provided an apparatus which projects an image produced on the cathode ray tube of a television receiver onto a screen through an optical system such as a lens. When the apparatus is installed, if the television receiver and projector are positioned in front of the screen, a viewer is hindered from seeing the screen. To avoid this obstacle, the projection of the reproduced picture onto the screen is carried out obliquely, for example, from the lower side of the screen as shown in FIG. 1.
In FIG. 1, the numeral 1 designates an image receiving tube such as a television receiver tube. An image reproduced thereon is projected upward through a lens 2 to a screen 3. This screen 3 is located in front of a viewer 4 in a plane generally perpendicular to the viewing direction 5 of the viewer 4. In this case, the angle between the viewing direction 5 and the projecting direction 6 (optical axis of the lens 2) is selected to be a predetermined angle .alpha..
When the image from tube 1 is projected obliquely onto the screen 3, as set forth above, a bow distortion is caused in a scanning line 7 of screen 3 as shown by the solid line in FIG. 2, in which a dotted line represents the desired scanning line.
Further, when the location of the projecting apparatus is changed, a twist distortion may be caused, for example, by the earth's magnetism, on the television receiver tube 1 and accordingly on on the scanning line 7 of screen 3 magnetism as shown by the solid line in FIG. 3, in which a dotted line represents the desired scanning line.
In order to remove such distortions, the prior art has used a vertical correction deflecting coil in addition to the normal vertical deflecting coil. To this correction deflecting coil a parabolic signal having a horizontal frequency is applied when the bow distortion is desired to be removed, and a sawtooth wave signal having a horizontal frequency is applied when the twist distortion is desired to be removed, or both such signals are applied to achieve the correction of a combination of such distortions. The use of such an additional deflecting coil has the undesirable effect of making the construction of the television apparatus more complicated and its cost more expensive.
A coupling transformer might also be used to correct for such distortion. The secondary side of the coupling transformer would be connected in series to the vertical deflecting coil and the parabolic signal and sawtooth wave signal would be applied to the primary side of the coupling transformer. In this case, unfortunately the apparatus becomes expensive due to the provision requirement of the coupling transformer.